Bond wire failures ( fusing) in integrated circuits with pulsed currents – an
analytical view.
1.0 Introduction: Bond wire fusing is not an uncommon phenomenon as far as failure mechanisms in ICs are concerned and may occur for a variety of reasons, almost all due to thermal heating of the wire. This brief treatment explores the analytical approach to estimating this mode of bond wire failure.
2.0 Conditions of Operation: In this discussion we are concerned with currents flowing through a bond wire which are pulsed in nature. The duty cycle is considered very small. The bond wire is considered to be short.
3.0 Fusing current expression: The expression for fusing current through a bond wire is shown below:
If =[ π D2 /4]
√[∆TCρ/tρ’]
D
= diameter of wire : (Cross-sectional area = π D2 /4)
∆T=
Melting temperature of wire above 25o C
C
= Specific heat of the wire
ρ
= density of wire
t
= pulse width
ρ’ = electrical resistivity
4.0
Material parameters: The table below shows the parameters
of interest for Aluminum, Gold and Silicon.
|
Parameter |
Alum |
Gold |
Si |
Units |
|
Specific
Heat, C |
0.856 |
0.128 |
0.7 |
J/DegKG |
|
Density,
ρ |
2.7 |
19.3 |
2.33 |
g/cm3 |
|
Elec.
Resistivity, ρ’ |
2.63E-6 |
2.1E-6 |
N/A |
Ohm-cm |
|
Melting
point above 25 Deg C, ∆T |
635 |
1020 |
1360 |
Deg
K. |
|
Thermal
conductivity, K |
2.37 |
3.1 |
1.5 |
W/cm-
Deg K |
|
Elec.
Resistivity at melting point |
11.1E-6 |
13.7E-6 |
- |
Ohm -
cm |
5.0
Calculations
for 1 mil gold wire: The following result is for a gold wire with,
D =
.001 inch or [ 25.6e-4 cm]
∆T
= 1020 Deg K
C =
0.128 J/Deg K.g
ρ
= 19.3 g/cm3
t =
pulse width = 250 ns.
ρ’= 2.1E-6
If = 354.9 Amps
If = 138.9 Amps ( using electrical Resistivity at melting point)
This result seems to indicate that if the pulse width is very
narrow, then the magnitude of the pulse could be enormous and the bond wire
would still be intact!
Ref: Electrical Overstress in integrated circuits by J.
Thomas May.